1. Field of the Invention
The present invention relates to a multilayer wiring board used for a high-frequency signal interface or the like, and more particularly to a multilayer wiring board to which a coaxial structure such as a connector is connected.
2. Description of the Related Art
In recent years, demands for reduction in device scale have increasingly grown in applications for various communication devices used in wireless or optical communication systems. Size reduction is also necessarily required for circuit boards for communication devices and high-frequency device modules. Therefore, the dimension of high-frequency connectors used as internal or external interfaces of communication devices is desired to be reduced. As one of connectors that meet such market demands, a small-sized connector such as a push-on SMP connector or a push-on SMPM connector has been put into practice. Furthermore, such a small-sized connector has heretofore been mounted directly on a circuit board. See, e.g., JP-A 2004-363593.
When a small-sized connector is mounted directly on a circuit board, a center conductor of the small-sized connector is brought into close contact with a high-frequency signal line of the circuit board. An outer conductor of the small-sized connector is brought into close contact with a ground electrode of the circuit board, which is formed on the same plane as the high-frequency signal line. When a ground layer is formed on a rear face of a circuit board as in the case of a microstrip line, a ground electrode formed on a front face of the circuit board is electrically connected to the ground layer formed on the rear face of the circuit board via through holes. In this manner, connection of high-frequency signals is established between the small-sized connector and the circuit board.
Meanwhile, what is called a multilayer wiring board is used in order to enhance the functionality of communication devices within a limited space. A multilayer wiring board is formed of a multilayered substrate, and a control signal line is also incorporated in the same substrate while the transmission characteristics of high-frequency signals are maintained. FIGS. 1A and 1B show a conventional example of a multilayer wiring board. As shown in FIGS. 1A and 1B, a multilayer wiring board 200 has a dielectric layer 500, a high-frequency signal line 400 formed on a surface of the dielectric layer 500 for transmitting high-frequency signals, and a ground layer 600. Additionally, the multilayer wiring board 200 has a control signal layer 900 for forming a control signal circuit that controls devices mounted on the multilayer wiring board 200. Thus, in view of size reduction and space saving of a communication device, it is important to effectively combine the multilayer wiring board 200 with a small-sized connector for a high-frequency device module.
In a high-frequency region, the characteristic impedance is likely to be discontinuous at a connection portion between a small-sized connector and a multilayer wiring board. Particularly, in a case where high-speed data signals over 10 Gb/s are transmitted, the signal band is so wide as to arise a serious problem of quality degradation of signal waveforms that is caused by discontinuous characteristic impedance. For example, in the conventional device illustrated in FIG. 5 of JP-A 2004-363593, the amount of signal reflection increases at a discontinuous point of the characteristic impedance, thus deteriorating the flatness of the transmission-frequency characteristics. As a result, the waveform quality of high-speed data signals is degraded.
Furthermore, in the case of the multilayer wiring board illustrated in FIGS. 1A and 1B, when a small-sized connector 100 is mounted onto the multilayer wiring board 200, a center conductor 300 of the small-sized connector 100 is connected to the high-frequency signal line 400 formed on the surface of the dielectric layer 500. Projecting portions 1000A of an outer conductor 1000 of the small-sized connector 100, which serves as a ground, are connected to ground electrodes 1200 formed on the surface of the dielectric layer 500. In order to exert a function of the high-frequency signal line 400, the ground electrodes 1200 on the dielectric layer 500, which also has the high-frequency signal line 400 formed thereon, are electrically connected to the ground layer 600 via a plurality of through holes 130.
However, each of those through holes 130 has a thickness corresponding to the thickness of the dielectric layer 500 and thus has a finite inductance. Therefore, provision of many through holes cannot sufficiently reduce a ground inductance at a connection portion between the coaxial structure (connector) and the circuit board. Accordingly, impedance discontinuity thus caused results in deterioration of frequency characteristics in transmission of high-frequency signals. Furthermore, for wide-band and high-frequency signals, such as high-speed data signals, variations in location and dimension of the through holes 130 being formed exert additional influences, make it difficult to obtain good frequency characteristics.